A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging the pattern using an ultraviolet (UV) radiation beam onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. Other lithographic apparatus may include interferometric and maskless lithographic patterning devices.
It has been proposed to immerse the substrate in the lithographic projection apparatus in a liquid having a relatively high refractive index, e.g., hydrocarbon liquid, water, etc., so as to fill a space between the final element of the projection system and the substrate. The liquid may be distilled water, although another liquid could be used. The description herein references a liquid. However, another fluid may be suitable, particularly a wetting fluid, incompressible fluid and/or a fluid with a higher refractive index than air, desirably a higher refractive index than water. The point of this is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid. (The effect of the liquid may also be regarded as increasing the effective numerical aperture (NA) of the system and also increasing the depth of focus.) Other immersion liquids have been proposed other than water. An organic fluid is one of these liquids being considered. An organic fluid has a higher refractive index than water and typically comprise a hydrocarbon, such as comprising an aromatic group, or a bi-aromatic group, for example decahydronaphthalene (also known as Decalin), a fluorohydrocarbon or a cubane dispersed in an organic solvent. Other proposed immersion liquids, include but are not limited to water with solid particles (e.g., quartz) suspended therein, particles in a fluid having the same or similar refractive index, and dispersed nanoparticles.
Submersing the substrate or substrate and substrate table in a bath of liquid means that there is a large body of liquid that must be accelerated during a scanning exposure. This requires additional or more powerful motors and the resulting turbulence in the liquid may lead to undesirable and unpredictable effects.
One of the solutions proposed is for a liquid supply system to provide liquid on only a localized area of the substrate and in between the final element of the projection system and the substrate using a liquid confinement system (the substrate generally has a larger surface area than the final element of the projection system). A liquid is supplied by at least one inlet IN onto the substrate, desirably along the direction of movement of the substrate relative to the final element, and is removed by at least one outlet OUT after having passed under the projection system. That is, as the substrate is scanned beneath the element in a −X direction, liquid is supplied at the +X side of the element and taken up at the −X side. A liquid is supplied via inlet IN and is taken up on the other side of the element by outlet OUT which is connected to a low pressure source. The liquid is supplied along the direction of movement of the substrate relative to the final element, though this does not need to be the case. Various orientations and numbers of in- and out-lets positioned around the final element are possible. One example is in which four sets of an inlet with an outlet on either side are provided in a regular pattern around the final element.
A twin or dual stage immersion lithography apparatus can be provided with two tables to support a substrate. Leveling measurements are carried out with a table at a first position, without immersion liquid, and exposure is carried out with a table at a second position, where immersion liquid is present. Alternatively, the apparatus has only one table.
As mentioned herein, a fluid such as a hydrocarbon is proposed as the immersion fluid, and has a high refractive index that is greater than existing immersion fluids, such as water.